Department of Psychiatry, University of California, San Francisco, San Francisco, CA 94121, USA. deborah.barnes@ucsf.edu

Abstract

At present, about 33·9 million people worldwide have Alzheimer's disease (AD), and prevalence is expected to triple over the next 40 years. The aim of this Review was to summarise the evidence regarding seven potentially modifiable risk factors for AD: diabetes, midlife hypertension, midlife obesity, smoking, depression, cognitive inactivity or low educational attainment, and physical inactivity. Additionally, we projected the effect of risk factor reduction on AD prevalence by calculating population attributable risks (the percent of cases attributable to a given factor) and the number of AD cases that might be prevented by risk factor reductions of 10% and 25% worldwide and in the USA. Together, up to half of AD cases worldwide (17·2 million) and in the USA (2·9 million) are potentially attributable to these factors. A 10-25% reduction in all seven risk factors could potentially prevent as many as 1·1-3·0 million AD cases worldwide and 184,000-492,000 cases in the USA.

The number of Alzheimer's disease (AD) cases that could potentially be prevented through risk factor reductions of 10% or 25% worldwide () and in the US () was estimated by multiplying current prevalence estimates by 0.90 and 0.75, respectively, and subtracting the revised number of attributable cases from the original number. These estimates assume that a causal relationship exists between the risk factor and AD and that the relative risk estimate is a good approximation of the impact of risk factor reduction. Therefore, the actual number of cases prevented could be higher or lower depending on the extent to which these assumptions are valid. In addition, the combined estimate assumes that the individual risk factors are independent and have an additive relationship. Because several of the risk factors examined are inter-related, the combined PAR estimates should be considered as maximums.

Together, we estimate that these seven potentially modifiable risk factors contribute to up to half of AD cases globally (17.2 million, ) and in the US (2.9 million) (). If the prevalence of all seven risk factors were 10% lower, we estimate that there would be as many as 1.1 million fewer AD cases globally and 184,000 fewer cases in the US; if risk factor prevalence were 25% lower, AD prevalence could potentially be reduced by up to 3.0 million cases globally and 492,000 in the US.